1. Field of the Invention
The present invention generally relates to a system and method for measuring properties of traveling webs of fibrous sheet material during manufacture and, more particularly, to a system and method for providing measurements of fibrous sheet materials using infrared measuring techniques.
2. State of the Art
In the manufacture of fibrous sheet materials such as paper and cardboard, important parameters for process control and product quality include basis weight and moisture content. Basis weight is defined as the weight per unit area of sheet material and is typically stated in units of grams per square meter. Dry basis weight refers to weight per unit area excluding moisture and, for paper products, is equivalent to the weight of dry material, primarily fibers, comprising a given area of a sheet. Moisture content refers to the amount of moisture per unit area of a sheet. These parameters are related by the fact that, for a given area of a sheet, basis weight minus moisture content equals dry basis weight.
It is well known that basis weight and moisture can be determined by laboratory physical tests. However, laboratory tests have several inherent drawbacks. One shortcoming is that substantial periods of time are required for sample acquisition and analysis. During those periods, production conditions may change sufficiently that the laboratory tests results, when available, are no longer representative of current manufacturing or product conditions. Another drawback of laboratory tests is that samples obtained for testing may not completely or accurately represent sheet material that has been produced.
To overcome the limitations of laboratory testing, various devices have been proposed for making measurements of properties of traveling webs of fibrous material "on-line", i.e., while a sheet-making machine is operating. On-line measurement devices have been proposed, for example, for measuring properties including basis weight, dry basis weight, moisture content, thickness, and transmissivity. In papermaking processes, however, it has proven difficult to design on-line measuring devices that operate accurately and for extended periods without undue repair and maintenance, and without causing down-time to sheet-making machines. The difficulties arise, in part, because modern papermaking machines are large and operate at high speeds. For example, some conventional papermaking machines are large enough to produce sheets that are 100 to 400 inches wide at rates from about 20 to 100 feet per second. Furthermore, on-line measurements in papermaking processes are often difficult to obtain because the environment around a papermaking machine may include a high concentration of water droplet and air-borne chemicals.
Among the on-line measurement devices that have been proposed are sensors that periodically traverse or "scan" traveling webs of sheet material. For example, U.S. Pat. Nos. 3,641,349; 3,681,595; 3,757,122; and 3,886,036 assigned to Measurex Corporation discuss basis weight gauges of the scanning type. Also, U.S. Pat. 4,289,964, assigned to Intec Corporation, suggests that beta ray gauges can scan across a traveling web in the cross direction to determine basis weight.
Despite numerous advantages of scanning gauges in sheet-making operations, many such gauges have encountered problems. Problems have arisen, for example, because moving parts in conventional scanning gauges require relatively frequent repair and maintenance. Also, conventional scanning gauges require relatively long periods to provide a "profile" of a traveling web. (In the sheet-making art, a profile is comprised of a succession of measurements at adjacent locations that, in total, extend completely across a traveling web.) For instance, conventional scanning gauges for detecting basis weight and moisture content of fibrous sheet materials normally require about thirty seconds to obtain a profile of a traveling sheet of average width. Thus, such conventional scanning devices are not well suited for high-speed papermaking machines whose control systems operate optimally when relatively larger numbers of measurements are provided over relatively shorter periods of time.
Specific examples of scanning gauges proposed by workers in the art include ones that detect the composition of sheet material by measuring the radiation absorbed from beams of infrared light or other radiation of known wavelength directed against a given area of the sheet material. That is, such devices operate in accordance with the general principal that the amount of radiation absorbed by sheet material at a particular wave length is a function of the composition of the sheet material; for example, the absorption of infrared light having a wavelength of about 1.5 microns provides an indication of the cellulose content of paper sheet material.
For purposes of on-line scanning of sheets of paper material, however, conventional infrared detectors are not completely satisfactory because they are relatively bulky, require mechanically protective enclosure of "heads", and function reliably only in environments that are cool, dry and of constant temperature. Due to such limitations, conventional infrared detectors are normally used at scanning speeds less than about fifteen inches per second. As mentioned above, such scanning speeds impose limitations upon data acquisition rates and, in turn, impose limitations upon control systems that employ the sensors. For modern high-speed papermaking machines, it can be said that the data acquisition rates reliably obtainable with conventional infrared scanners are too slow for optimum benefits to be derived from computerized control systems.
As further background to the present invention, it is useful to generally describe a typical papermaking process. Generally speaking, a papermaking process begins when a slurry of fibers and water, called raw stock, is spread onto a supporting wire mesh from a reservoir called a head box. The wire mesh supports the fibers while allowing substantial drainage. The web is passed through a press section that squeezes water from the web and then through a dryer section to evaporate water from the web. After the dryer section, the web passes through calendar rollers and, usually, through a scanner and onto a reel.
The portion of a papermaking process prior to the dryer section is often referred to as the "wet end". It can be appreciated that on-line measurements at the wet end are desirable because such measurements, if acted upon promptly, may minimize wastage by indicating needed process changes before substantial quantities of substandard paper are produced. On the other hand, wet end measurements are difficult to make because of the high water content, usually about sixty percent, of paper webs at this stage and because of severe environmental conditions.